Now, though the fella behind @TheDisneyBlog hated Fast Pass (and I have yet to get round to asking him why, although I’m very curious), I found it a thoroughly useful and excellent invention in 2004 when I first used it at WDW.
The concept is brilliantly simple: the queue is too long right now, so you get an hour-long slot (for rides) or performance time (for shows) to come back for, leaving you with a very short wait compared to the standby lines.
In practice most of the Fast Pass tickets are gone by 11am, but if you plan your park visit carefully – it does require a certain amount of military precision although there’s still arguably room for spontaneity – you’ll get hold of ones for the rides you most want to go on. For me the key Fast Passes to bag will be for Splash Mountain, Mission Space and Soarin’, I reckon (if they all do FP, which I believe they do).
No system is perfect, but as a way of helping you plan your day around the park without having to account for very long queues, it’s very helpful.